1. Technical Field
The invention mainly relates to a process for manufacturing a generally aerodynamically-shaped structure in which adjacent channels or grooves are formed. More precisely, the manufacturing process according to the invention uses the SPFDB (SuperPlastic Forming and Diffusion Bonding) technique.
The invention also relates to an aerodynamic structure obtained using this process.
One of the preferred applications of the invention is in the aeronautical industry. In particular, the process according to the invention is used to make a fixed or mobile flap substructure provided with internal grooves, for a leading edge. These grooves may be used particularly for controlling the airflow or for defrosting outer surfaces.
However, the invention can also be used in other technical fields, to make all types of ducts integrated into a structure or all types of panels stiffened by the partitions separating the grooves.
2. State of the Art
In order to make aircraft safe and to improve their performances, the leading edge of wings, tail fins, control surfaces, and jet air intakes, are usually equipped with devices that prevent the formation of ice or control the air flow over the aerodynamic surfaces of these various elements.
Document U.S. Pat. No. 3,933,327 shows a deicing device of a jet pod, in which the leading edge comprises two skins leading warm air towards radial passages that open out onto a honeycomb sandwich structure. This structure is perforated to carry warm air towards the outside skin of the pod in order to prevent the formation of ice. It also enables acoustic attenuation of noise emitted by the flow. The two skins and the warm air passages are made by the use of conventional manufacturing processes used in aeronautics.
Document WO-A-98/47761 shows the leading edge of an aircraft wing formed from two parallel skins connected to each other by stiffeners, in order to define a partitioned structure capable of resisting aerodynamic forces. The spaces delimited between the skins and the stiffeners form communication areas for air drawn in through perforations passing through the outer skin. Air drawn in from the inside of the leading edge reduces turbulence on the contour of the leading edge. This has the effect of reducing drag on the wing and consequently improving aircraft performances. This type of structure is made using a number of special tools, and particularly jigs for positioning and assembling the stiffeners on the two skins, but this tends to increase manufacturing costs and times.
Document U.S. Pat. No. 6,050,523 describes a leading edge structure that comprises a substructure made of a composite material and a titanium skin glued to this substructure. In the direction of the wing, the substructure comprises circular ducts facing perforations made in the skin. It is very difficult to make such a structure since it involves the use of different techniques to form the skin with the required profile and to obtain the substructure from a composite material. A combination of these two techniques for manufacturing and assembly of the skin on the substructure require long and complex operations.
Document EPA-0 376 371 describes a structure designed to avoid the formation of ice on the aerodynamic surfaces of an aircraft such as the wings and flaps, by the use of a warm fluid circulating in ducts arranged along the chord of a section and from one part to the next, over the entire length of the wing. The manufacturing process used to make the leading edge of the wing and the deicing ducts at the same time uses a set of moulds that reproduces the outer shape of the leading edge and the shape of the ducts. According to this process, the plates are prepared and then partially assembled by a metallurgical operation. They are then preformed and placed between a male mould that defines the outer contour of the structure and a female mould that defines the shape of the ducts. A heat source then enables the plates to deform plastically under the action of a gas to match the shape of the female mould.
Document U.S. Pat. No. 2,690,002 relates to a process for making a thin structure for a heat exchanger. The structure is made by placing one of the two plates on top of the other. These two plates are separated at the locations at which the ducts will be formed, using a material that will prevent their assembly during a subsequent hot rolling operation. When the said operation has been done and the plates are assembled, a fluid is injected under pressure between the plates to form the ducts symmetrically on each side of the plates. If it is desired that the ducts should only deform on one of the faces of the structure, the plate opposite this face is made thicker such that it is sufficiently stiff to prevent it from deforming. This technique, that is based on a free expansion of the plates, does not use a mould to form the different parts of the structure, unless a particular prismatic section, for example a triangular shape, is required for the ducts.
Both of the manufacturing processes described in documents EP-A-0 376 371 and U.S. Pat. No. 2,690,002 use a pressure cycle to make the structure, which requires either the use of forms and mating forms, or a symmetric structure to balance forces while the ducts are being formed.
Presentation of the Invention
The invention mainly relates to a manufacturing process for a grooved structure for which the innovative design solves at least some of the problems that arise when making structures using the processes according to prior art.
More precisely, the purpose of the invention is a particularly simple and inexpensive process for making a grooved structure without the use of complex tooling or procedures.
According to the invention, this objective is achieved by using a process for manufacturing at least one metallic structure comprising a smooth outer skin, an inner skin and partitions connecting the said skins by delimiting adjacent cells from each other, the said process being characterized in that it comprises the following main steps:
placement of at least one stack of three metallic plates T1, T2, T3 superposed in a mould comprising a surface complementary to the smooth outer skin of the structure to be made, with the insertion of an anti-diffuser between the plates in regions that are not to be assembled;
heating of the stack of plates and execution of a plate diffusion welding cycle, outside the said regions that are not to be assembled;
partial formation of the cells and shaping of the structure by making a first phase in a pressure cycle, consisting of applying a first pressure in the first regions R1 delimited between a first plate T1 adjacent to the said mould surface and an intermediate plate T3 in the said stack and a second approximately constant pressure independent of and lower than the first pressure, on the face of the third plate T2 opposite the said surface and in the second regions R2 delimited between the third plate T2 and the intermediate plate T3;
final conformation of the cells and the outer skin by carrying out a second phase in the pressure cycle consisting of simultaneously applying an approximately equal pressure in the first regions R1 and the second regions R2, and on the said face of the third plate T2.
The process according to the invention is based on the fact that the pressures applied firstly in the first regions R1 corresponding to one out of every two cells in the manufactured structure, and secondly in the second regions R2 (the other cells) on the face of the stack corresponding to the inner skin of the manufactured structure, are independent. During the first phase of the pressure cycle, the cells are partially formed by inflation of the first regions. During the second phase of the pressure cycle, the cells and the outer skin are brought into their final shape to create the required smooth nature. This is done by progressively applying the stack in contact with the mould while maintaining the pressure on all the cells.
In a first preferred embodiment of the invention, the first pressure applied in the first regions R1 gradually increases up to a predetermined value, during about a first half of the duration of the first phase of the pressure cycle, and then remains constant until the end of the said first phase, when the first pressure goes down very quickly to a value equal to the value of the second pressure applied in the second regions R2 and on the said face of the third plate T2.
Advantageously, the predetermined value of the first pressure applied in the first regions R1 during the first phase of the pressure cycle is approximately equal to four times the second pressure then applied in the second regions R2 and on the said face of the third plate T2.
According to another aspect of this preferred embodiment, the total duration of the first phase of the pressure cycle is about 60 minutes.
According to another aspect of this preferred embodiment, the second pressure applied in the second regions R2 and on the said face of the third plate T2 remains approximately constant and equal to about one bar during the first phase of the pressure cycle.
According to another aspect of the preferred embodiment of the invention, the pressures in the first and second regions R1, R2 and on the said face of the third plate T2 increase regularly during the second phase of the pressure cycle until a final determined pressure is obtained, preferably equal to about 9 bars.
Advantageously, the duration of the second phase of the pressure cycle is approximately equal to the duration of the first phase of the pressure cycle.
According to a particular embodiment of the invention, two stacks of plates are placed simultaneously in the mould and the said stacks are separated during a preliminary phase of the pressure cycle, during which a low and identical pressure is applied for a very short time in the first and second regions R1, R2, and on the said face of the third plate T2, the preliminary phase of the pressure cycle preceding the first phase of the pressure cycle.
In this particular embodiment, the preliminary phase of the pressure cycle advantageously lasts for about one minute and the pressure applied during this preliminary phase is about 1 bar.
Specifically, the first pressure is advantageously applied through at least one distribution channel towards which the first regions R1 are open.
Advantageously, the face of the first plate T1 facing the intermediate plate T3 is partially machined along strips that will be welded to the intermediate plate, such that their thickness is equal to the thickness of the intermediate plate before the stack is placed in the mould. Thus, the thickness of the outer skin of the manufactured structure is approximately constant and its outer surface is perfectly smooth.
According to another aspect of the invention, the width of the cells is made approximately equal to five times the height of the said cells.
Finally, another purpose of the invention is a metallic structure comprising a smooth outer skin, an inner skin and partitions connecting the said skins delimiting adjacent cells between them, characterized by the fact that it is obtained using the previously defined process.